1. Field of the Invention
The present invention relates to an automatic wafer orienting apparatus.
2. Description of the Prior Art
In the manufacture of semiconductor elements it is generally necessary, in plural steps, to exactly register the pattern on a wafer with the pattern of a photomask (mask registration) for pattern printing.
Such mask registration has been achieved manually by observation of the wafer pattern and the photomask pattern under a microscope, but in recent years various automatic mask registration methods utilizing photoelectric detection are coming into use.
In a manual mask registration, the registration of the mask can be achieved if a registration pattern on the wafer is properly positioned in the viewing field of the microscope, and the precision of registration is generally within .+-.150 .mu.m in consideration of the general dimension of the viewing field of the microscope. For orienting a wafer within said precision of registration there is already disclosed, in the U.S. Pat. No. 3,982,627, a wafer orienting apparatus for driving and rotating a wafer thereby orienting the wafer while utilizing a lateral edge and an orientation cut thereof as the guide.
In an automatic mask registration wherein the registration is achieved by photoelectric detection signals from a wafer pattern and a photomask pattern, it is generally necessary to use special automatic alignment marks which have to be incorporated in the wafer, thus wasting a certain number of semiconductor elements.
The size of such automatic alignment mark, or namely the number of semiconductor elements to be wasted, can be represented as a function of the precision of wafer registration, and can be reduced as the precision is improved.
Also in case of preparing a photomask with a repeater, the reticule has to be exchanged for incorporating the automatic alignment marks in place of the semiconductor element patterns, and the eventual position error between said patterns and the automatic alignment marks resulting from such reticule exchange is an important cause for error in automatic registration.
As explained in the foregoing, a size reduction of the automatic alignment mark will improve the yield of semiconductor elements in a given area of the wafer, and it will be rendered possible to dispense with the reticule exchange and to significantly improve the precision of automatic registration if the automatic alignment marks can be made smaller so as to be incorporated in each semiconductor chip.
For these reasons the size reduction of the automatic alignment marks is being raised as a major target in the manufacture of the semiconductor elements.
In various automatic registration methods already proposed, the wafer is generally subjected to a pre-alignment or pre-orientation with a peripheral edge and an orientation cut thereof as the guide, and then is subjected to an automatic mask registration by means of a photoelectric detecting mechanism which is designed to have a coverage corresponding to the precision of said pre-alignment and which detects the positional aberration between the automatic alignment marks of the wafer and the photomask to control the wafer or the photomask by the detection signal. Consequently an improvement in the precision of pre-alignment allows reduction of the coverage of the photodetection and the size of the automatic alignment marks.
In the prior art technology, the pre-alignment is for example achieved by driving a peripheral edge of the wafer with a drive roller to rotate said wafer and to disengage the drive roller when the orientation cut of the wafer is brought to a determined position, or by providing, in addition to the above-mentioned drive roller, another drive roller rotating in an opposite direction whereby the rotating force of said rollers to the wafer becomes balanced when the orientation cut of the wafer becomes simultaneously engaged with said two rollers.
In such pre-alignment methods it has been impossible to stably achieve a pre-alignment precision not exceeding .+-.10 .mu.m since the pre-alignment is performed by stopping the wafer in a rotational movement by means of the orientation cut thereof.
Also such known aligning apparatus, requiring a considerable mechanism, cannot be mounted directly on the wafer chuck but is generally provided outside thereof. Consequently, in order to obtain an elevated pre-alignment precision within .+-.10 .mu.m, it is required to realize and maintain a positional precision of several micrometers between the wafer chuck and the alignment apparatus positioned outside said wafer chuck which is repeatedly displaced for printing operations. Because of such difficulties the precision of pre-alignment in the prior art has remained in a range of .+-.150 .mu.m.